The term damascene when used in connection with integrated circuit wiring, refers to the fact that a layer has been inlaid within a supporting medium, as opposed to being covered by it. The main advantage of this approach to wiring is that it is highly cost effective relative to conventional wiring. The word `damascene` is derived from the city of Damascus where inlaid jewelry of this general format was first produced. In the present invention, the area for receiving the inlaid conductor is formed in two separate steps so the process is more correctly termed `dual damascene` but we shall use `damascene` and `dual damascene` interchangeably.
Copper is the metal most widely used in damascene technology, because of its excellent electrical conductivity. Unfortunately, copper is also a rapid diffuser, particularly into silicon where its presence plays havoc with the semiconducting properties of that material. Accordingly it is necessary to surround all copper wiring with a diffusion barrier to isolate it from the remainder of the integrated circuit. Where no electrical contact between the wiring and the circuit is required, the barrier may be an insulator but when electrical contact must be made the barrier must also be an electrical conductor.
An example of damascene wiring (shown in schematic cross-section) is given in FIG. 1. The wiring is composed of two parts, both of which extend downwards from the surface of dielectric 1. The main part of the wiring (in the sense that it is always present) is trench 3. The second part is cavity, or via hole, 2 that is present whenever contact needs to be made between the wiring and some other layer (not shown in this example). Once 2 and 3 have been formed (generally in two steps but not necessarily, as mentioned below) their sidewalls are coated with a barrier layer material such as tantalum, tantalum nitride, or titanium nitride. They are then overfilled with copper, following which the surface is planarized so that no copper remains on the original surface 5 of dielectric layer 1.
Unfortunately, because it is much softer then the dielectric, the copper gets removed somewhat faster and the result is dishing of the copper across the mouths of trenches 3. Dishing is undesirable because it reduces the conductive cross-section of the wire. Additionally, in order to minimize the amount of dishing, the planarization process is often terminated somewhat prematurely so a thin layer of copper gets left behind on the surface of the dielectric.
At the completion of planarization, a second barrier layer is laid down to isolate the top surface of the wiring. Silicon nitride is widely used for this purpose. While silicon nitride is an effective barrier layer, it does have a tendency to react with the copper because of the latter's reaction to the ammonia that is present during the former's deposition.
Thus, two improvements over the state of the art are needed. These are a trench filling and planarization technique that does not lead to dishing or copper residues and a barrier layer that does not react adversely with the copper. The present invention addresses both these issues.
In a search of the prior art we were unable to find any references that teach the method of the present invention. Some references were, however, found to be of interest. For example, Carey (U.S. Pat. No. 5,173,442 December 1992) describes a method for forming a damascene connector using relatively few process steps. The vias are first etched using a combination of hard and soft masking. The latter gets consumed as the via forms and, once it is gone, etching of both the trench and the via proceed simultaneously, as determined by the hard mask alone.
Krishnan et al. (U.S. Pat. No. 5,451,551 September 1995) teaches a damascene process in which a first barrier layer is deposited on the trench walls and the trench is then only partially filled with copper. The trench is then overfilled with material of a second barrier layer so that, after CMP, the top surface of the copper is protected by said second barrier layer. This patent is a divisional of an earlier patent (U.S. Pat. No. 5,380,546 January 1995) by the same authors and is very close to same.